Token ring networks can include as many as 250 end stations, typically connected together at one or more concentrators or hubs. In those instances where two or more hubs are used to provide enough ports to accommodate the end station the hubs are interconnected by wire or fiber segments or trunks designated "Ring In" and "Ring Out" on token ring networks.
During normal ring operation, these segments pass signals between the hubs they interconnect. In the event of a segment failure, it is critical that hardware or software at the hub detect the failure condition and wrap Ring In to Ring Out to bypass the failed segment. This converts the dual ring to a single working ring when a wrap occurs at both ends of the failed segment. The IEEE 802.5 token ring standard describes a method for accomplishing the needed wrap.
Wrapping a faulty segment is only half of the problem. Once the fault has been corrected the ring must be unwrapped if another fault occurs and needs to be bypassed. The IEEE 802.5. Token Ring Standard sets forth a procedures for unwrapping and merging a wrapped segment (or trunk) in a dual ring token passing network. However, the procedure requires at least the use of one full function media access control (MAC) device or adapter connected to the wrapped segment.
U.S. patent application Ser. No. 08/381,695, filed on Jan. 31, 1995 and assigned to the same assignee as this application discloses an application specific integrated circuit (ASIC) isolation and restoration device which is substantially lower in cost than a full IEEE 802.5 Token Ring MAC device. This device in combination with novel methods of operation is capable of achieving end results equal to or comparable to results achieved by use of a full MAC when operated as recommended in the IEEE 802.5 Standard.
The ASIC device while capable of accomplishing some of the end results achieved by a full MAC device is, however, not capable of engaging in token passing routines, a critical function performed by a full MAC device when implementing the recovery from a wrapped segment recommended in the IEEE 802.5 standard.